In electric arc welding of pipe sections, it is normal practice to use one or more electrodes fed through drive rolls into a groove between adjoining pipe sections. The groove is filled with a granular flux for protecting the weld bead formed by the melted electrode and the melted workpieces at the bottom of the groove. The welding process produces a weld bead of molten metal covered by the molten slag formed from the granular flux. The electrode, whether solid wire or cored wire, includes an outer layer of copper for facilitating the electrical contact between the steel of the electrode, and the power lead from a welding power source. The resistivity of copper is quite low so the wire passing through the contact tip of the welding torch receives current from the contact tip without large heat loss in the torch itself and without arcing between the wire and the contact tip. Consequently, it is very important that the outer layer of the advancing weld metal be copper; however, during the feeding operation the copper layer on the wire often creates flakes and particles of pure copper that are mechanically removed from the surface during the forced feeding of the welding wire through the liner into the torch and through the torch contact tip into the welding operation. These flakes or particles of copper have a substantial mass and commingle with the granular flux covering the welding operation.
Particles of copper dispersed in the flux are melted by the molten slag and often pass through the molten slag into contact with the weld bead. The bead metal cools and solidifies at approximately 2800° F.; however, the copper in the molten slag remains molten and migrates through the slag to the surface of the solidified weld bead. This molten copper has a low surface tension and tends to migrate into the solidified weld metal at the grain boundaries. Copper in the grain boundaries of the weld bead can cause subsequent cracking, referred to in the industry as “copper cracking.” Copper cracking is a specific example of a generic phenomenon known as liquid metal embrittlement (LME). LME involves cracking of a solid substrate caused by a liquid metal. Copper cracking becomes even more pronounced as the granular flux is reused, thereby accumulating greater amounts of copper flakes or particles dispersed through the granular flux. Thus, as flux is reused, there is an increased amount of copper particles to migrate through the molten slag over the top of the solidified weld metal, thus increasing the tendency for subsequent copper cracking of the weld metal bead. To reduce this tendency, the welding industry sometimes uses only new granular flux during each pass in the pipe welding process. This is costly and results in a substantial waste of excellent, effective granular flux. Another procedure for reducing copper cracking is to assure no abrasive action against the wire as it is being fed. This requires expensive and frequent attention to refurbishing the total welding equipment, often during the use of the equipment for pipe welding. Both of these procedures involve the objective of reducing the amount of copper in the granular flux. They are expensive and time consuming procedures requiring routine and expensive intervention.
Also, the use of copper free wire can lead to excessive tip heating and electrical erosion, particularly at the high currents used in pipe welding.